This invention relates to an apparatus for generating electric signals for simulating the output of a measuring apparatus sensitive to a physical variable and providing an output electric signal, indicative of the measured physical variable. The physical variable may be temperature and thus the apparatus in question may be a thermocouple, a resistance thermometer or the like.
Devices which measure physical variables and which have an electric output have been used for a long time. In particular such devices have been used for a long time in connection with the measurement of temperature and reference may be made to thermocouples and resistance thermometers.
With particular reference to thermocouples these devices have been used in laboratories and in industrial plants as sensitive elements for measuring temperature and for a long time apparatus has been used which simulates the electric signals coming from such thermocouples, this apparatus being used to calibrate or adjust apparatus to operate in response to signals generated by the thermocouples.
It has been proposed to utilise, as an apparatus for simulating an electric signal coming from a thermocouple, a potentiometric bridge. Such a potentiometric bridge can be considered to comprise an emf (electro-motive force) generator and means for providing an adjustable measured emf, derived from that emf generator. It has been proposed to connect such a potentiometric bridge to a thermocouple temperature measuring instrument, the bridge taking the place of a thermocouple itself, so that the temperature measuring instrument can be checked and so that, if necessary, the calibration of the temperature measuring instrument can be adjusted. In skilled hands such a potentiometric bridge may produce an emf which is equal to the emf which a thermocouple, being simulated by the bridge, would generate at a specific desired temperature.
However, it is to be appreciated that the method of ensuring that the potentiometric bridge generates an emf equal to that generated by a specific thermocouple at a desired temperature is complicated, for the reasons that are set out below.
There are various types of thermocouple that are in common use, these different types of thermocouple utilising different types of junction. Each specific type of thermocouple has its own characteristic output curve and thus each type of thermocouple produces a different emf at any specific given temperature and the correspondence between the temperature and output emf is not easy to express algebraically. Consequently a person utilising a potentiometric bridge to simulate a thermocouple must utilise suitable conversion table to find the value of the emf that the thermocouple in question should produce at the desired temperature so that the potentiometric bridge may be operated to produce the corresponding emf. Alternatively the operator of the potentiometric bridge must have a number of scales for use with the bridge each scale being calibrated directly in terms of temperature, the appropriate scale being used in conjunction with the bridge with any secific type of thermocouple.
A further complication is that at the present time two different temperature measuring scales are in common use, namely the Celsius or centrigrade scale and the Fahrenheit scale. Consequently, in view of the difficulties described above, it will be necessary for an operator of the potentiometric bridge to have two sets of conversion tables or alternatively two sets of reading scales.
A further factor that adds to the complication of utilising a potentiometeric bridge is due to the fact that when a thermocouple is in use one junction of the thermocouple, normally termed the "cold" junction is kept at a constant reference temperature which is normally 0.degree. C., namely the temperature of melting ice. The other junction of the thermocouple is at the temperature which is to be measured and the scales or tables mentioned above will be prepared on the assumption that the cold junction is at the reference temperature. However, in practice, some thermocouples may be operated with the "cold" junction at a temperature which is not 0.degree. C. and if this is the case, before the emf for any particular measured temperature taken from the table is simulated by the potentiometric bridge it is necessary to subtract from that emf the emf shown on the table at the temperature that the "cold" junction actually has, the difference between these two emf's being simulated by the potentiometric bridge.
Yet a further difficulty associated with the use of a potentiometric bridge is that, before a potentiometric bridge can be used it is usually necessary to carry out some preliminary checking operations, known as "standardization", to ensure that the emf actually being generated by the potentiometric bridge is correct. These preliminary operations are relatively long, and normally have to be carried out periodically during the use of the potentiometric bridge. The reason that these preliminary operations have to be carried out is that a potentiometric bridge usually utilises a cell or battery as the primary source of emf, and the emf generated by such a source can vary during operation of the potentiometric bridge by virtue of polarization within the battery or cell, or by virtue of changes in temperature of the cell or battery, or for other causes. Of course, in due time such a battery will become exhausted and then the emf produced by such a battery will fall.
It is to be appreciated that whilst the above discussion has concentrated on the simulation of the emf generated by a thermocouple, similar problems also occur in connection with the simulation of the output signals generated by resistance thermometers, since again each resistance thermometer will have a characteristic curve which must be tabulated to some extent. It is to be appreciated, of course, that similar problems occur generally whenever it is desired to simulate the electric output of a device which is sensitive to a physical variable and which provides an electric output which is indicative of the instantaneous value of that physical variable. This is especially the case when the output is not linearly dependent upon the physical variable to be measured.